FIG. 8 is a diagram of a correction control system in a conventional fuel injector. In the control system, a supply voltage VB of a supply terminal 11 is input to a microcomputer 13 in an electronic control unit (hereinafter, “ECU”) via a supply voltage input circuit 12.
When the supply voltage VB is low, the microcomputer 13 provides a field effect transistor (hereinafter, “FET”) driver 15 with a pulse having such a waveform that elongates the on-time period of an FET 14. As a result, a coil current flows through a solenoid 16 for a longer time to elongate a fuel injection time. When the supply voltage VB is high, to the contrary, the fuel injection time is shortened to keep the fuel injection amount unchanged. Immediately after the FET 14 is turned from ON to OFF, the current flowing through the solenoid 16 is redirected to a zener diode 18 via a diode 17. As a result, the drain voltage of the FET 14 is equalized to the voltage of the zener diode 18, which consumes power to halt fuel injection.
FIG. 9 is a diagram of a constant current control system in a conventional fuel injector. In the control system, the supply voltage VB of the supply terminal 11 is detected by a supply voltage detector 21. The coil current is detected at a current detection resistor 22 by a current detector 23 additionally provided for current detection. The microcomputer 13 and a constant current driver 24 control the coil current not to vary even if the supply voltage VB varies.
The conventional art for correcting the fuel injection amount by detecting variations in the supply voltage is disclosed, for example, in Japanese Patent Application Laid-open No. S58-28537. The conventional art for correcting the fuel injection amount by detecting the supply voltage and the drive current flowing through the solenoid is disclosed, for example, in Japanese Patent Application Laid-Open No. 2002-4921.
In the correction control system based on the supply voltage VB as shown in FIG. 8, however, the resistance of the coil in the solenoid 16 fluctuates with increased temperature of the coil, to change the coil current even if the supply voltage VB is unchanged. Therefore, it is difficult to correct the fuel injection amount accurately.
In contrast, the constant current control system shown in FIG. 9 can control the coil current unchanged even if the temperature of the coil varies. In this case, however, it causes an increase in the number of components due to the complex controller and an increase in software processing.
FIG. 10 is a diagram of an internal circuit of the current detector 23 shown in FIG. 9. FIG. 11 is a diagram for explaining the influence of offset voltages on current detection. As shown, the drive current generates a voltage of the current detector 23 (an offset voltage between the current detection resistor 22 and the current detector 23: Vinoffset); an offset voltage of an operational amplifier 25 in the current detector 23 (Vopoffset); and an offset voltage of an analog to digital (hereinafter, “A/D”) converter 26 in the microcomputer 13 (Vadoffset). The offset voltage between the current detection resistor 22 and the current detector 23 (Vinoffset) and the offset voltage of the operational amplifier 25 in the current detector 23 (Vopoffset) increase according to the amplification factor of the operational amplifier 25.
Thus, as shown in FIG. 11, the input voltage of the A/D converter 26 (Vadin) includes an additional offset component voltage (Vadinoffset) other than a voltage generated by an inherent drive current component (Vadini). The offset component voltage (Vadinoffset) occupies a proportion not negligible to deteriorate the accuracy of the current detection and interfere with precise fuel injection control.
The present invention is made in view of the above problems, and its object is to provide a fuel injection method for precise correction of the fuel injection amount by eliminating the offset component that are generated when detecting the current flowing through the solenoid for fuel injection.